1. Field of the Invention
The present invention relates to a method of printing on a mixed sequence of media sheets, using a printer with a print head that is adjustable in height relative to a print surface that supports the media during printing, wherein each of the media sheets have different specifications for an admissible height range for the print head relative to the print surface.
2. Description of Background Art
In many printers, e.g. ink jet printers, the print head is disposed at a short distance above the surface of the media sheets that are supported on a print surface, e.g. a conveyer belt, a platen or the like. For simplicity, the term “height” is used here to define the distance between the print surface and a surface of the print head that faces the print surface in a direction normal to the print surface, and it is assumed that the print head is disposed “above” the print surface, although the present invention is not limited to printers where the print surface is oriented horizontally.
In order to obtain a high print quality, it is desired that the distance between the print head and the surface of the media sheets is as small as possible. For example, in an ink jet printer, a small distance between the print head and the media sheets is desired in order to minimize the distance that the ink droplets have to travel and thereby to minimize the amount of aberration of the ink droplets. On the other hand, the print head must, of course, not collide with the media sheets, so that there should always be a certain safety distance between the surface of the media sheets and the print head. This safety distance will depend upon the thickness tolerances of the media sheets, their tendency to cockle or to form wrinkles and the like, so that a lower limit of the admissible range for the height of the print head is derivable from the specifications of the individual types of media sheets. The upper limit of this admissible range may also depend upon the media type, because certain properties of the media sheet, such as their capability to permit the ink droplets to spread over the sheet surface and/or to absorb ink in the interior of the sheet material, have an influence on the tolerable aberration of the ink droplets and consequently on the distance that the ink droplets have to travel.
The media types that are to be used for printing are specified in the print job or print jobs that are to be processed with the printer. The media type may not only vary from print job to print job, but even a single print job may specify that different media types are to be used for different pages of the document to be printed. Consequently, there may be cases where a mixed sequence of media sheets of different types is scheduled for printing. In that case, depending upon the thickness of the different media sheets and on the media specifications, it may be necessary to change the height adjustment of the print head in the interval between the printing periods for two successive sheets.
As the height adjustment of the print head takes a certain amount of time, considerable losses in productivity may occur when the height of the print head has to be re-adjusted frequently.
The losses in productivity are particularly pronounced in duplex printing. In a typical duplex printer, the duplex loop accommodates a plurality of sheets, so that a set consisting of a certain number of sheets is moved past the print head a first time for printing an image on the first side of the sheets, and then these sheets will be returned via the duplex loop and will be moved past the print head a second time in order to form an image on the second side. When this set of sheets contains different media types, each adjustment operation of the print head that has been performed during printing on the first side of the sheets has to be repeated when these sheets return from the duplex loop. Duplex printing may be established batch wise by filling the duplex loop with sheets and then printing each sheet twice, or in a more productive mode of interweaving sheets of a plurality of duplex and/or simplex print jobs. For example, in the interweaving mode, empty sheets alternate with sheets that have already been printed on one side.